The invention relates to the separation of gases from hydrocarbon gas mixtures. In particular, the invention relates to the separation of carbon dioxide from hydrocarbons. The separation is carried out using hydrocarbon-resistant membranes, and is useful in natural gas processing plants and the like.
Polymeric gas-separation membranes are well known and are in use in such areas as production of oxygen-enriched air, production of nitrogen from air, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from air or nitrogen.
The preferred membrane for use in any gas-separation application combines high selectivity with high flux. Thus, the membrane-making industry has engaged in an ongoing quest for polymers and membranes with improved selectivity/flux performance. Many polymeric materials are known that offer intrinsically attractive properties. That is, when the permeation performance of a small film of the material is measured under laboratory conditions, using pure gas samples and operating at modest temperature and pressure conditions, the film exhibits high permeability for some pure gases and low permeability for others, suggesting useful separation capability.
Unfortunately, gas separation in an industrial plant is seldom so simple. The gas mixtures to which the separation membranes are exposed may be hot, contaminated with solid or liquid particles, or at high pressure, may fluctuate in composition or flow rate or, more likely, may exhibit several of these features. Even in the most straightforward situation possible, where the gas stream to be separated is a two-component mix, uncontaminated by other components, at ambient temperature and moderate pressure, one component may interact with the membrane in such a way as to change the permeation characteristics of the other component, so that the separation factor or selectivity suggested by the pure gas measurements cannot be achieved.
In gas mixtures that contain condensable components, for example C3+ hydrocarbons, it is frequently, although not always, the case that the mixed gas selectivity is lower, and at times considerably lower, than the ideal selectivity. The condensable component, which is readily sorbed into the polymer matrix, swells or, in the case of a glassy polymer, plasticizes the membrane, thereby reducing its selective capabilities. Carbon dioxide is also known to swell or plasticize many membrane materials. As a result of these effects, a technique for predicting mixed gas performance under real conditions from pure gas measurements with any reliability has not yet been developed.
A good example of an application in which membranes have difficulty delivering and maintaining adequate performance is the removal of carbon dioxide from natural gas. Natural gas provides more than one-fifth of all the primary energy used in the United States, but much raw gas is xe2x80x9csubqualityxe2x80x9d, that is, it exceeds the pipeline specifications in nitrogen, carbon dioxide and/or hydrogen sulfide content. In particular, about 10% of gas contains excess carbon dioxide. Membrane technology is attractive for removing this carbon dioxide, because many membrane materials are very permeable to carbon dioxide, and because treatment can be accomplished using the high wellhead gas pressure as the driving force for the separation.
However, since carbon dioxide readily sorbs into and interacts strongly with many polymers, and most natural gas contains at least some C3+ hydrocarbons, the expectation is that the gas components will have a swelling or plasticizing effect, thereby adversely changing the membrane permeation characteristics. These issues are discussed, for example, in J. M. S. Henis, xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranes,xe2x80x9d Chapter 10 of D. R. Paul and Y. P. Yampol""skii, Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. This reference gives upper limits on various contaminants in streams to be treated by polysulfone membranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation of aromatics, 25% saturation of olefins and 11xc2x0 C. above paraffin dewpoint (pages 473-474).
In the past, cellulose acetate, which can provide a carbon dioxide/methane selectivity of about 10-20 in gas mixtures at pressure under favorable conditions, has been the membrane material of choice for this application, and about 100 plants using cellulose acetate membranes are believed to have been installed. Nevertheless, cellulose acetate membranes are not without problems. Natural gas often contains substantial amounts of water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. However, contact with liquid water can cause the membrane selectivity to be lost completely, and exposure to water vapor at relative humidities greater than only about 20-30% can cause irreversible membrane compaction and loss of flux. The presence of hydrogen sulfide in conjunction with water vapor is also damaging, as are high levels of C3+ hydrocarbons. These problems are presented in more detail in U.S. Pat. No. 5,407,466, columns 2-6, which patent is incorporated herein by reference.
Thus, the need remains for membranes that will provide and maintain adequate performance under conditions of exposure to organic vapors, particularly C3+ hydrocarbons, in conjunction with high concentrations of acid gas and water vapor that are commonplace in natural gas treatment.
Films or membranes made from fluorinated polymers having a ring structure in the repeat unit are known. For example:
1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclose various perfluorinated polymers having repeating units of perfluorinated cyclic ethers, and cite the gas-permeation properties of certain of these, as in column 8, lines 48-60 of U.S. Pat. No. 4,910,276.
2. A paper entitled xe2x80x9cA study on perfluoropolymer purification and its application to membrane formationxe2x80x9d (V. Arcella et al., Journal of Membrane Science, Vol. 163, pages 203-209 (1999)) discusses the properties of membranes made from a copolymer of tetrafluoroethylene and a dioxole. Gas permeation data for various gases are cited.
3. European Patent Application 0 649 676 A1, to L""Air Liquide, discloses post-treatment of gas separation membranes by applying a layer of fluoropolymer, such as a perfluorinated dioxole, to seal holes or other defects in the membrane surface.
4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patent also discloses comparative data for membranes made from perfluoro(2-methylene-4-methyl-1,3-dioxolane) polymer (Example XI).
5. A paper entitled xe2x80x9cGas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylenexe2x80x9d (I. Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133 (1996)) discusses the free volume and gas permeation properties of fluorinated dioxole/tetrafluoroethylene copolymers compared with substituted acetylene polymers. This reference also shows the susceptibility of this dioxole polymer to plasticization by organic vapors and the loss of selectivity as vapor partial pressure in a gas mixture increases (FIGS. 3 and 4).
Most of the data reported in the prior art references listed above are for permanent gases, carbon dioxide and methane, and refer only to measurements made with pure gases. The data reported in item 5 indicate that even these fluorinated polymers, which are characterized by their chemical inertness, appear to be similar to conventional membranes in their inability to withstand exposure to propane and heavier hydrocarbons.
The invention is a process for separating carbon dioxide from a gaseous hydrocarbon in a gas mixture. Such a mixture might typically, but not necessarily, be encountered during the processing of natural gas, of associated gas from oil wells, or of certain petrochemical streams. The mixture is typically a multicomponent mixture, containing the gaseous hydrocarbon from which it is desired to separate carbon dioxide, as well as at least one other gaseous hydrocarbon and/or other component such as nitrogen, hydrogen sulfide or water vapor, for example.
The process is carried out by running a stream of the gas mixture across a membrane that is selective for carbon dioxide over the hydrocarbon from which it is to be separated. The process results, therefore, in a permeate stream enriched in carbon dioxide and a residue stream depleted in carbon dioxide. The process can separate carbon dioxide from methane, carbon dioxide from ethylene, carbon dioxide from ethane, carbon dioxide from C3+ hydrocarbon vapors, or any combination of these, for example.
The process differs from previous carbon dioxide/hydrocarbon separation processes in that:
(i) the membranes are able to maintain useful separation properties in the presence of organic vapors, particularly C3+ hydrocarbon vapors, even at high levels in the gas mixture,
(ii) the membranes are able to withstand high partial pressures of carbon dioxide, and
(iii) the membranes can recover from accidental exposure to liquid organic compounds.
To provide these attributes, the membranes used in the process of the invention are made from a glassy polymer or copolymer. The polymer is characterized by having repeating units of a fluorinated, cyclic structure, the ring having at least five members. The polymer is further characterized by a fractional free volume no greater than about 0.3 and preferably by a glass transition temperature, Tg, of at least about 100xc2x0 C. Preferably, the polymer is perfluorinated.
In the alternative, the membranes are characterized in terms of their selectivity before and after exposure to liquid hydrocarbons. Specifically, after exposure of the separation membrane to a liquid hydrocarbon, for example, toluene, and subsequent drying, the membranes have a post-exposure selectivity, for carbon dioxide over the desired gaseous hydrocarbon, that is at least about 60%, 65% or even 70% of a pre-exposure selectivity for carbon dioxide over the gaseous hydrocarbon, the pre- and post-exposure selectivities being measured with a test gas mixture of the same composition and under like conditions.
The selective layer is again made from an amorphous glassy polymer or copolymer with a fractional free volume no greater than about 0.3 and a glass transition temperature, Tg, of at least about 100xc2x0 C. The polymer is fluorinated, generally heavily fluorinated, by which we mean having a fluorine:carbon ratio of atoms in the polymer of at least about 1:1. Preferably, the polymer is perfluorinated. In this case, however, the polymer need not incorporate a cyclic structure.
In a basic embodiment, the process of the invention includes the following steps:
(a) bringing a multicomponent gas mixture comprising carbon dioxide, a gaseous hydrocarbon, and a third gaseous component into contact with the feed side of a separation membrane having a feed side and a permeate side, the membrane having a selective layer comprising:
a polymer comprising repeating units having a fluorinated cyclic structure of an at least 5-member ring, the polymer having a fractional free volume no greater than about 0.3;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in carbon dioxide compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in carbon dioxide compared to the gas mixture.
In the alternative, a basic embodiment of the process includes the following steps:
(a) bringing a multicomponent gas mixture comprising carbon dioxide, a gaseous hydrocarbon, and a third gaseous component into contact with the feed side of a separation membrane having a feed side and a permeate side, the membrane having a selective layer comprising a polymer having:
(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;
(ii) a fractional free volume no greater than about 0.3; and
(iii) a glass transition temperature of at least about 100xc2x0 C.;
and the separation membrane being characterized by a post-exposure selectivity for carbon dioxide over the gaseous hydrocarbon, after exposure of the separation membrane to liquid toluene and subsequent drying, that is at least about 65% of a pre-exposure selectivity for carbon dioxide over the gaseous hydrocarbon, as measured pre- and post-exposure with a test gas mixture of the same composition and under like conditions;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in carbon dioxide compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in carbon dioxide compared to the gas mixture.
Particularly preferred materials for the selective layer of the membrane used to carry out the process of the invention are amorphous homopolymers of perfluorinated dioxoles, dioxolanes or cyclic alkyl ethers, or copolymers of these with tetrafluoroethylene. Specific most preferred materials are copolymers having the structure: 
where x and y represent the relative proportions of the dioxole and the tetrafluoroethylene blocks, such that x+y=1.
A second highly preferred material has the structure: 
where n is a positive integer.
Contrary to what would be expected from the data presented in the Pinnau et al. Journal of Membrane Science paper, we have unexpectedly found that membranes formed from fluorinated cyclic polymers as characterized above can withstand exposure to C3+ hydrocarbons well enough to provide useful separation capability for gas mixtures that include C3+ hydrocarbon vapors. This resistance persists even when the C3+ hydrocarbons are present at high levels, such as 5%, 10%, 15% or even more.
A particularly important advantage of the invention, therefore, is that the membranes can retain selectivity for carbon dioxide even in the presence of streams rich in, or even essentially saturated with, C3+ hydrocarbon vapors. This distinguishes these membrane materials from all other membrane materials previously used commercially for such separations.
The membranes are also very resistant to plasticization by carbon dioxide, even at very high carbon dioxide partial pressures.
In addition, the membranes are resistant to contact with liquid hydrocarbons, in that they are able to retain their selectivity for carbon dioxide after prolonged exposure to liquid toluene, for example. This is a second beneficial characteristic that differentiates the processes of the invention from prior art processes. In the past, exposure of the membranes to liquid hydrocarbons frequently meant that the membranes were irreversibly damaged and had to be removed and replaced.
These unexpected and unusual attributes render the process of the invention useful in situations where it was formerly difficult or impractical for membrane separation to be used, or where membrane lifetimes were poor.
Because the preferred polymers are glassy and rigid, an unsupported film of the polymer may be usable in principle as a single-layer gas separation membrane. However, such a layer will normally be too thick to yield acceptable transmembrane flux, and in practice, the separation membrane usually comprises a very thin selective layer that forms part of a thicker structure, such as an asymmetric membrane or a composite membrane. The making of these types of membranes is well known in the art. If the membrane is a composite membrane, the support layer may optionally be made from a fluorinated polymer also, making the membrane a totally fluorinated structure and enhancing chemical resistance. The membrane may take any form, such as hollow fiber, which may be potted in cylindrical bundles, or flat sheets, which may be mounted in plate-and-frame modules or formed into spiral-wound modules.
The driving force for permeation across the membrane is the pressure difference between the feed and permeate sides, which can be generated in a variety of ways. The pressure difference may be provided by compressing the feedstream, drawing a vacuum on the permeate side, or a combination of both. The membrane is able to tolerate high feed pressures, such as above 200 psia, 300 psia, 400 psia or more.
As mentioned above, the membrane is able to operate satisfactorily in the presence of C3+ hydrocarbons at high levels. Thus the partial pressure of the hydrocarbons in the feed may be close to saturation. For example, depending on the mix of hydrocarbons and the temperature of the gas, the aggregate partial pressure of all C3+ hydrocarbons in the gas might be as much as 10 psia, 15 psia, 25 psia, 50 psia, 100 psia, 200 psia or more. Expressed as a percentage of the saturation vapor pressure at that temperature, the partial pressure of hydrocarbons, particularly C3+ hydrocarbons, may be 20%, 30%, 50% or even 70% or more of saturation.
The carbon dioxide partial pressure may also be relatively high, such as 25 psia, 50 psia, 100 psia or above.
The process of the invention typically provides a selectivity, in mixtures containing multiple hydrocarbons including a C3+ hydrocarbon vapor, for carbon dioxide over methane of at least about 5, even at high carbon dioxide activity. Frequently, the carbon dioxide/methane selectivity achieved is 10 or more, and may be as much as 15 or more, even in the presence of significant concentrations of C3+ hydrocarbons.
The membrane separation process may be configured in many possible ways, and may include a single membrane unit or an array of two or more units in series or cascade arrangements. The processes of the invention also include combinations of the membrane separation process defined above with other separation processes, such as adsorption, absorption, distillation, condensation or other types of membrane separation.
The scope of the invention in this aspect is not intended to be limited to any particular gas streams, but to encompass any situation where a gas stream containing carbon dioxide and a gaseous hydrocarbon is to be separated. The composition of treatable gas streams varies widely, and the individual components may be present in any quantities. Thus, feed gas streams may contain just a few percent carbon dioxide, or 90% carbon dioxide or more. The gas may contain a single hydrocarbon component, such as methane, ethane or propane, or a mix of numerous hydrocarbons, such as C1-C8 hydrocarbons or heavier. The third component of the gas stream may be a second hydrocarbon, an inert gas, a second acid gas, water vapor or any other component.
It is envisaged that the process will be particularly useful as part of a natural gas processing train. Pipeline specification for natural gas is usually no more than about 2% carbon dioxide, but raw gas frequently contains more than 2% carbon dioxide and not infrequently contains 10% carbon dioxide or more. The process of the invention enables gas that is out of specification with respect to carbon dioxide to be brought to pipeline specification. Furthermore, since the membranes used are able to withstand other contaminants in the gas, the carbon dioxide removal steps may be performed upstream of other gas treatments, if required. This provides greater flexibility in plant design and operation than is often possible using prior art carbon dioxide separation methods.
It is an object of the present invention to provide a membrane-based process for separation of carbon dioxide from a gaseous hydrocarbon.
It is an object of the invention to provide a membrane-based process for treating out-of-specification natural gas.
Additional objects and advantages of the invention will be apparent from the description below to those of ordinary skill in the art.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.